Up to now, a power conversion apparatus is known which on-off controls a semiconductor switching element, such as an IGBT, for supplying power to a load, such as a motor. As such a conventional technology, a technology which is disclosed in Japanese Unexamined Patent Application Publication No. 2009-219294 is available.
The gate driving circuit of a power conversion apparatus based on this conventional technology transmits a gate driving signal from a microcomputer through two pulse transformers provided inside of an IC chip, including: a low side circuit which outputs a gate driving signal transmitted by one pulse transformer to a low side IGBT; a high breakdown voltage nMOS which performs potential conversion of a gate driving signal for a high side IGBT that has been transmitted through the other pulse transformer; and a high side circuit which outputs the gate driving signal having been subjected to potential conversion by the high breakdown voltage nMOS to the high side IGBT. And, on the basis of the driving signal (gate driving signal) outputted from the microcomputer, a current is supplied to the primary winding of the pulse transformer to thereby generate a voltage in the secondary winding, which is compared with the reference voltage to take out a signal.
Generally, the gate driving signal transmitted has a pulse width of approx. 1 ns to 2 ns, in other words, belongs to a frequency band of 1 GHz or so, thereby a transmission circuit including a receiver CMP capable of high speed transmission as shown in FIG. 1 is used.
In FIG. 1, when a gate driving signal for the primary side is inputted from a microcomputer to a primary winding N1 of a pulse transformer T through a driver DRV, this gate driving signal is transmitted to a secondary winding N2 of the pulse transformer T to be received by a differential receiver CMP. And, from an output terminal of the receiver CMP, a signal is outputted to the gate of an IGBT on the high side or low side as a gate driving signal on the secondary side. The two gate driving signals for the high side and the low side are transmitted through the respective two pulse transformers, however, FIG. 1 shows only one of them.
Thus, the IGBT is operated with a gate driving signal belonging to a frequency band of 1 GHz or so, and especially the low side IGBT is operated in a floating state with respect to the ground potential of a main power supply (herein, the main power supply refers to a power supply for supplying power to a load). Then, the receiver CMP for receiving a gate driving signal transmitted through the pulse transformer T is defined to be a current driven differential type one the characteristic impedance of which is matched, thereby providing a circuit configuration which is capable of high speed transmission.
The gate driving signal for an IGBT is generally a pulse signal produced by a PWM method, and such pulse signal is required to be raised or lowered at high speed. To meet such requirement, the receiver CMP is operated without being saturated, thereby being adapted to be operated at high speed. Resistors R1 to R4 shown in FIG. 1 play a role to set the voltages inputted to the input terminals of the receiver CMP at a level for always holding the operation of the receiver CMP in a non-saturated state.
The resistor R1 is connected between a non-inverted input terminal of the receiver CMP and a control power supply Vcc; the resistor R2 is connected between the non-inverted input terminal of the receiver CMP and a second ground potential point GND2; the resistor R3 is connected between an inverted input terminal of the receiver CMP and the control power supply Vcc; and the resistor R4 is connected between the inverted input terminal of the receiver CMP and the second ground potential point GND2. Further, the potential at the second ground potential point GND2 generally provides a potential of the emitter of the IGBT driven. Therefore, in the gate driving circuit for the high side IGBT, the second ground potential point GND2 is generally in a floating state with respect to the ground potential of the main power supply.